Two classical methods of measuring density of a fluid include: 1) measuring the weight of fluid in a flask of fixed volume; and 2) employing the buoyancy of a density float for the assessment of fluid density. Both of these methods require collection of large samples from a fluid-processing device such as a pipeline or reactor for off-line measurements. For a given solution, density relates to the solute concentration of the solution. Although the density measurement is not specific to what solute is in the solution, these two density measurement methodologies and others to be described later have been used as a means to assess solute concentration.
A mechanical device based on resonance has been available to measure density of a fluid sample or that of a flowing fluid on-line. The device has a hollow U-tube with its two ends fixed on a heavy base. The fluid can be infused to fill the U-tube or made to flow along the tube. By measuring the frequency that the U-tube resonates, one then determines the mass of fluid in the U-tube. Since its volume is fixed, the mass is converted to the fluid density. This mechanical density measuring system (MDMS) has high sensitivity and reproducibility in the dynamic measurement of fluid density.
As a fourth density measurement method, one measures the sound velocity of fluid for the determination of the compressibility and density of fluid. Krivitski, in U.S. Pat. Nos. 5,453,576 and 5,685,989 describes an apparatus and method for measuring several hemodynamic parameters by using a sound velocity sensor. The ultrasound transducer is excited to emit a pulse of ultrasound. After its passage through a fluid medium such as the blood, a receiving transducer senses the ultrasound pulse. A protocol to compare the excitation and receiving ultrasound signals determines the transmission time through the blood and subsequently its sound velocity. The information contained in the '576 and the '989 patent is incorporated by reference as though cited in its entirety. When their device is used to measure blood density change for the computation of blood volume, a linear approximation of a non-linear relationship is employed to convert the sound velocity to the density of the blood. Furthermore, the device of the '576 patent has limited sensitivity so as to require the imposition of a large change in blood density for accurate assessment of blood volume.
The system patented by Schneditz in U.S. Pat. No. 5,830,365 also utilizes sound velocity for the measurement of transmission time delay through the blood and then its total protein concentration. A clinical protocol to change the ultrafiltration rate as a patient undergoing hemodialysis treatment is described to produce the change in density, which is assessed through a sound velocity monitor. An equation is deduced to compute from the change measurement the blood volume circulating in the patient. The monitor to measure density is about one order of magnitude less sensitive than that provided by the MDMS or our density/solute monitor. As a result, the application of Schneditz's method to measure blood volume is limited to cases that the change in sound velocity being imposed through the clinical protocol is large.
The fifth density measurement method employs the attenuation due to the absorption of radioactivity by the fluid as a means to assess its density. Approval by regulatory agency is required for this method.